Recently, there has been developed and commercialized so far such an apparatus which employs an optical disc (Writable disc or DRAW) on which a digital data can be recorded one time and an optical disc (optical magnetic recording disc or the like) from which a digital data can be erased as an external storage or to record other digital data.
By the way, when such apparatus is developed and commercialized, a digital conversion system (Channel Modulation) to substantially convert a digital data to a recorded pattern on the disc becomes a serious problem. The digital modulation is a system to convert a raw data series to a certain data which can be matched with characteristics of media (discs). One of the necessary conditions in this system is that an energy of a servo frequency band in a low frequency range has to be small within a spectrum of a modulation signal. If the energy of the servo frequency band in the low frequency range is high, it affects a servo loop as an external disturbance, obstructing a servo from being effected accurately as it is expected. This condition is also taken into consideration for a disc such as a compact disc that is used only for the playback. Particularly, in the case of a recordable disc apparatus, upon recording, a laser light source must be driven by a modulation signal so that in the modulation system with much low frequency components, an energy applied as an external disturbance becomes rather large to disturb very much the servo from being carried out precisely.
In view of such aspect, this invention is made and provides an optical disc recording and reproducing apparatus by which even in a modulation system in which an energy of a low frequency component is relatively large, a focusing servo and a tracking servo can be stably carried out in the recording mode.